Abstract A modified Weibull function accurately described the seasonal leading shoot growth of Pinus resinosa Ait. as a function of growing degree day (GDD) sums for five seasons at one station in Minnesota. 41.4°F was a significantly better threshold temperature than the commonly cited 40°F from which to calculate GDD. When tested on data from six other stations, predictive ability was variable, ostensibly because of microclimatic differences between the reference weather stations and the red pine sites. With the exception of one unusual case, variability was greatly reduced by empirically assigning a daily mean temperature bias of no more than ± 1.5°F to the reference stations. As is, the GDD model seems capable of providing sufficiently reliable estimates of shoot growth maturity to identify relatively warm, moderate, and cool growing regions to improve the timing of a variety of silvicultural applications related to plant maturity. With calibration of reference stations, the model may be sufficiently sensitive to aid analysis of thermal variability of forested microclimates, to estimate the effects of water stress, or other activities requiring more precise estimates. Forest Sci. 31:913-925.